1. Technical Field
The present disclosure relates to a transmission and reception apparatus for digital signals and method thereof.
2. Description of the Related Art
It is known in prior art the use of galvanically isolated interfaces between a transmitter and a receiver, in which the signals are wirelessly transmitted. The digital signal transmission must be of the high bitrate type for transmitting data between one chip and another, while ensuring galvanic isolation. Typical bitrate values are 50-100 Mbit/s, while the isolation is in the range of 2-5 kV.
Various apparatuses for providing the galvanic isolation currently exist.
One of these consists in using an integrated transformer. The latter is made by means of a pile structure in which the secondary winding generally consists of the metal layer arranged at the bottom level, the primary winding of the metal layer arranged at the upper level and the isolation between the two windings consists of various dielectric layers arranged between the two metal layers, the thickness of which depends on the desired level of isolation. Alternatively, the secondary winding may be made of the metal layer arranged on the upper level, the dielectric layer is inserted thereon and finally the primary winding is made of an additional metal layer. The isolation transformer is formed on the receiver die. The driver die contains the transmitter which, through the bonding, is connected to the primary of the isolation transformer. The secondary of the transformer is connected to the receiver which processes the transmitted signal. The data flow may also occur from the secondary to the primary, if a two-way channel is desired. The signals must be appropriately processed by means of a modulation technique in order to transfer information through the isolated interface. This type of component allows to obtain a high bitrate, good reliability and isolation capacity. However, this configuration has the addition of processing steps for obtaining the transformer (e.g., for increasing the thickness of the isolating layer in order to achieve the required degree of isolation), as well as the use of bonding wires for connecting the transmitter of the driver die to the transformer itself.
Another galvanic isolation apparatus, described in US Patent Publication No. 2008/0311862, comprises a system based on a transmission of wireless type. The two chips are reciprocally assembled and the isolation is obtained by means of an interposed insulating layer. The electromagnetic coupling is made by means of a pair of coils, in particular by means of the magnetic field produced by the current which flows over the transmission coil. The thickness of the upper chip (e.g., the transmitter chip) is reduced by lapping in order to maximize the coupling between the two coils. The signals are appropriately processed by means of a modulation technique in order to transfer information through the isolated interface. This configuration has the advantage of requiring neither additional processing steps nor bonding wires between the two chips. However, it has the disadvantage of a low coupling coefficient and high variability thereof which depends on the thickness tolerance after lapping the die, the thickness tolerance of the insulating layer and the alignment tolerance between the two chips.
A further galvanic isolation apparatus, again described in US Patent Publication No. 2008/0311862, comprises a system based on a transmission of wireless type. In this case, the two chips are assembled side-by-side and the isolation is obtained by means of an insulating layer placed below the two chips. The electromagnetic coupling is made by means of a pair of coils, in particular by means of the magnetic field produced by the current which flows over the transmission coil. The signals are appropriately processed by means of a modulation technique in order to transfer information through the isolated interface. This configuration also has the advantage of requiring neither additional processing steps nor bonding wires between the two chips. However, it has the advantage of a highly variable, lower coupling coefficient as compared to the previous structure, according to the alignment tolerance between the two chips and the distance tolerance between the two chips.
Regardless of the structure used for making the coupled inductors, today there exist two main approaches for transmitting a signal through the channel.
A first technique is of the narrowband type, in which the signal is transmitted on a carrier. Thereby, a high signal/noise ratio, or SNR, may be obtained to the detriment of a high circuit complexity due to the use of radio frequency circuits, such as an oscillator, a low-noise amplifier, a mixer, a filter, etc.
A second technique is of the wideband type, in which wideband pulses (UWB Pulses) are transmitted. Thereby, the circuit complexity of the transceiver is reduced, but the signal/noise ratio is worse.
A transmission technique which may be implemented with a relatively simple system is the on-off keying (OOK) technique, in which information is encoded in the form of transmitted signal absence-presence. FIG. 1 shows a block diagram of the system with a transmitter 100 on the die 101, a receiver 200 on the die 201, a galvanic isolator 300, an input signal Sin to the transmitter 100 and an output signal Sout from the receiver.
If a narrowband approach is used, the input signal to the isolator 300 enables an oscillator which generates the OOKW carrier. The signal is amplified and processed in the receiver in order to extract the envelope.
If a wideband approach is used, a pulse TP is generated at each (positive and negative) wave-front of the input signal to the isolator 300.
The signal is amplified and processed in the receiver to extract the polarity thereof. Thereby, for example, a positive wave-front is decoded if the received pulse has positive polarity, otherwise a negative wave-front is decoded. FIGS. 2 and 3 show the main waveforms of the signals in a system of this type, with narrowband and wideband techniques, respectively.
Such a system, i.e., a system based on the recognition of the signal presence-absence, which allows to obtain an isolated channel through which the transmission of a digital signal characterized by two levels (high and low) is possible, is referred to as “two-level isolated digital channel”. The previously mentioned applications use isolated digital channels in which the synchronization signal (clock) may be transmitted in addition to the data signal itself. The simplest, but more expensive solution is to employ two channels: one for transmitting the clock and one for transmitting the data. In actual fact, there are more advantageous solutions, which use appropriate modulation techniques to transmit the clock and data on a single channel: ASK modulation, FSK modulation, etc. ASK modulation allows the simultaneous transmission of the clock and data in a relatively simple system, by implementing the narrowband approach. FIG. 4 shows a block diagram of a system, which allows the transmission and reception of the clock and data on a single channel, e.g., based on ASK modulation.
The Clock signal, at interface input, enables an oscillator which generates an ASKW carrier, the amplitude of which is modulated by the Data signal. For example, if the transmitted data is “zero”, the peak-to-peak amplitude of the carrier is equal to Vdd/2 (where Vdd is the supply voltage of the transmitter); if the transmitted data is “one”, the peak-to-peak amplitude of the carrier is equal to Vdd.
The received signal is amplified and processed in order to extract the envelope. Such an envelope is compared, by means of a comparator, with two thresholds: a low threshold VthL and a high threshold VthH. If the amplitude of such a signal is comprised between the low threshold VthL and the high threshold VthH, the demodulated datum is “zero”, while if the amplitude is higher than the high threshold VthH, the demodulated datum is “one”. At the same time, the envelope must exceed one of the two thresholds to extract the clock. FIG. 5 shows the main waveforms of a system which uses the ASK modulation technique, i.e., the input clock signal ClockIN to the transmitter, the input data signal DataIN to the transmitter, the carrier ASKW, the envelope ASKENV, the output clock signal ClockOUT from the receiver, and the output data signal DataOUT from the receiver.
In order to ensure a good interference immunity, the amplifier gain and the threshold values VthL and VthH may be appropriately fixed. In particular, once the values of the thresholds VthL and VthH have been fixed, the gain is chosen so that the amplitude of the input signal to the comparator, when zero is received, is equal to (VthH+VthL)/2. The received signal amplitude depends on the transmitted signal amplitude and coupling coefficient of the coils. Therefore, in the case of the pile or planar structure, the amplitude of the received signal is highly variable because, as previously mentioned, the coupling coefficient depends on the assembly quality. Therefore, the circuitry of the receiver is complicated, e.g., by inserting a variable gain amplifier (VGA) and a gain control loop to compensate for such a variability. Furthermore, before transmitting the data itself, the carrier with a known amplitude is transmitted for a time interval so that the system may appropriately adjust the VGA gain. Unfortunately, such an increase of circuit complexity of the receiver thwarts the use of the ASK modulation technique, which is indeed chosen for its simplicity.